Introduction

Every particle, when dispersed in water, has an electrostatic surface charge.  Fundamentally, electrostatic charges are the result of the arrangement, type, and concentration of ions at the surfaces of the particles.  They are also functions of the pH of the interparticle solution, and the type and concentration of ions and molecules suspended in the interparticle fluid. 

Electrostatic forces can be attractive (systems of two different types of particles, one with positive and one with negative surface charges), or repulsive (systems where all particles have like charges, either positive or negative.) 

The electrostatic charge is specified by the value of the zeta potential ( ζ ) of suspended particles.  Zeta potential, in turn, is determined by measuring the electrophoretic mobility of colloidal particles in suspension as they flow in a controlled electric field.

Each different type of material is characterized by an IsoElectric Point (IEP) which defines the value of the pH of the interparticle fluid at which the net surface charge on the particles is zero.  IEP values are functions of the major cation(s) at the surfaces of the each different type of particle in suspension. 

IsoElectric Point & pH

Each powder, when dispersed in water, has a net surface charge which is a function of the pH of the carrier fluid.  As the pH changes, net surface charges change.  There is a value of pH for each type of powder at which the net surface charge at the particle surface is zero.  This pH is known as the IsoElectric Point (IEP).  At all pH values higher than a powder's IEP, electrostatic surface charges will be negative.  At all pH values lower than a powder's IEP, electrostatic surface charges will be positive.

The way to remember this is that OH^- ions, which dominate at high pHs, have negative charges.  When negatively charged hydroxyl ions are in high concentration, particle surfaces will be negatively charged.  At the other end of the pH spectrum, where pH values are low and H⁺ ions concentrations are high, particle surfaces will be positively charged.  IEPs for all of the different powders cover a wide range of pH values.

When a deflocculated (well-dispersed) suspension is desired, the pH of the suspension should be maintained at values higher or lower than (that is, away from) the IEPs of all suspended particle types.  If all particles are to have the same sign of surface charge so they'll repel, this is the requirement. 

Sometimes, this is difficult to achieve.  For example, silica has an IEP in the pH 2-3 range, while alumina has an IEP in the pH 9-10 range.  When silica and alumina particles are suspended together, for all particles to be positively charged, the suspension must be set to a pH below the IEP of silica.  Such solutions will be very acidic.  On the other hand, for all particles to be negatively charge, the suspension must be set to a pH above the IEP of alumina.  Such solutions will be very basic.  Additives, which will be discussed in a later section, can be used to accomplish this at pH values between the IEPs of silica and alumina -- which means at more neutral, common pH values.

Zeta Potential

The zeta potential of a particle is defined as the electrostatic potential at the shear plane of the fluid surrounding each particle.  A thin layer of fluid will always travel with each particle.  Fluid inside this shear plane is sometimes referred to as the particles water hull.  Some say that the thickness of the water hull is a fixed distance -- the same for all particles.  The thickness of this hull appears, rather, to vary with suspension properties and even, possibly, shear conditions.  At the shear plane, which is a small, finite distance from the surface of each particle, fluid will be sheared away as the particle flows through the suspension.  Any ions and fluid inside this shear plane will travel with the particle.  Any ions and fluid outside this plane will be sheared away as the particle moves through the suspension.  When flow stops, the ions and fluid in the bulk interparticle fluid surrounding each particle will re-equilibrate with each particle and the species within its shear plane.

Since zeta potential is defined as the electrostatic potential at the shear plane, it is NOT the surface potential of the particles.  It IS the potential a minute distance away from the surface.  It is the effective potential of the particle because it is a measure of the potential at the effective surface of the particle as it flows through the fluid.  This is the case because zeta potentials are measured as particles flow.  We may not know precisely how much fluid and how many ions or molecules flow with each particle, but we know that however much fluid and however many ions and molecules actually travel with each particle, measured zeta potentials will be the values at the surface of the fluid, ions, and molecules traveling with the particle.  As mentioned above, some define the fluid that travels with a particle as its water hull.  If you prefer this terminology, the zeta potential is the potential at the surface of each particle's water hull.

Relative Strength of Electrostatic Forces

Relatively speaking, electrostatic forces of attraction or repulsion are stronger than van der Waals forces of attraction.  This depends, of course, on the value of electrostatic forces.  At the IEP, where net charges are zero, van der Waals forces of attraction will pull particles together and the system will be flocculated.  At pH values away from the IEP where net electrostatic charges are positive or negative, electrostatic charges will dominate and van der Waals forces of attraction, though present, will be masked and hidden.

Hydrophobic effects are generally stronger than electrostatic forces.  When hydrophobic additives are used in suspensions, anionic (negatively charged) additives will still coat particle surfaces, even when those surfaces are also negatively charged.

The order of strength of the various forces are:  Hydrophobic forces are generally strongest, electrostatic forces are next, and van der Waals forces of attraction are weakest.  Van der Waals forces of attraction are always present, but they are usually masked and are invisible due to the dominance of electrostatic and hydrophobic forces.

Additive Effects

Some additives to ceramic suspensions supply inorganic soluble ions and some supply hydrophobic organic molecules or polymers.  Additive ions will typically be attracted to oppositely charged surface sites.  Singly charged positive ions, such as sodium and potassium (Na⁺ and K⁺), are relatively large ions which will be weakly attached to negative surface sites, where they will spend only short residence times.  They come and go rather quickly.  More highly charged ions, such as magnesium and calcium (Mg⁺⁺ and Ca⁺⁺) are smaller, which means it is easier for them to attach more strongly and maintain longer residence times at the negatively charged sites.  Even more highly charged ions, such as aluminum (Al⁺⁺⁺) are smaller yet with even higher charge concentrations.  They attach even more strongly and stay for even longer residence times than the other larger, lower charged ions.

When shear is applied to suspensions (and depending on the level of applied shear), all ions may be kicked off particle surfaces and out into the interparticle soup.   When shear is removed, they will re-equilibrate and relocate onto and near the particle surfaces.

When hydrophobic organic additives, which stick more strongly to surfaces than most cations, are used in ceramic suspensions, the order of addition of the additives also is important.  This complicates the issue terribly. 

For example, consider two cases:

1 -- When hydrophobic organic additives are the first additions to suspensions, particle surfaces will be coated by these organic additives.  If soluble cations are then added, the cations will attach to available negatively charged surface sites which may include both negative particle surface sites and negative hydrophobic organic additive sites.  The types and locations of the negative sites can be dramatically different in the presence of the organic additives than on the original, clean surface sites.  When the organic additives are anionic polyelectrolytes (which are long chain polymers with lots of negative sites along their length) they will present many negatively charged sites to attract soluble cations.

2 -- When relatively clean particle surfaces are exposed first to soluble cations, the cations will be attracted to the particles' negatively charged surface sites.  If hydrophobic organic additives are then added to the suspension, the organic additives will be pushed out of the water and onto the surfaces where they may cover, trap, and mask the adsorbed cations that were associated with the original negatively charged surface sites. 

When the concentrations of hydrophobic additives and soluble ions are the same in these two cases, the resulting suspensions will have different effective surface charges due to the arrangements of soluble ions and hydrophobic additives on the particles. 

It is likely, and very probable, that these two systems will exhibit different rheological properties due to different additive arrangements on the particle surfaces.  When shear of varying intensities is applied to each, different amounts of cations and organics may be dislodged from the surfaces to join the interparticle soup  -- thereby changing the surface concentrations and net electrostatic charges of the surfaces.

Under conditions of high intensity dispersion (HID), it is possible to strip all ions and molecules from the surfaces of the particles.  When the HID is removed, ions and molecules will re-equilibrate and the resulting equilibrium arrangements will differ from the arrangements that occurred when the particles were originally tuned as described in cases 1 and 2 above.  It is likely that after HID, the two suspensions will behave similarly, but both will behave differently than their original behaviors.  Instead of two possible behaviors to deal with, HID will give a third possible behavior.

One thing should be noted about HID.  If it is sufficiently intense to strip all additives from particle surfaces, without chopping organic polymers into pieces,  it will produce an equilibrium that can be reproduced by subjecting the suspension once again to HID.  The suspensions produced by following the procedures listed in 1 and 2 above will only remain stable if shear is minimized so as to not disturb the original arrangements of additives.  Under shear (lower intensities than HID), both suspensions 1 and 2 will change with time as surface additives undergo rearrangement.

What is the correct order to add the various additives?  It depends -- there is no fixed answer.  Which of the two systems can be sheared without altering the surface arrangements?  That depends, too.  Which order of addition will produce surface arrangements that will not change if the suspension is subjected to shear?  That depends as well.

When fundamentally different types of additives are all going into a ceramic suspension, it is necessary that each different order of addition be tested to determine the proper solution for your particular suspension and your particular additive package.  Or simply add all the additives simultaneously into the suspension while it is undergoing high intensity dispersion.

In all cases, pH should be adjusted first.  Then, other additives can be introduced.  The proper order of addition of the other additives must be experimentally determined.

Practical Considerations

Some particles, like clays and kaolins, may have regions of both negative and positive surface charges simultaneously.  Over much of the pH range, clays will have positive edges and negative surfaces.  This means that within pure clay and kaolin systems, flocculating conditions can be achieved simply because edges are attracted to surfaces, and surfaces are attracted to edges.  In most other single-powder systems, pH alone (except at the IEP) will produce like charges on all particles (and deflocculating conditions). 

When two or more particle types are combined in a suspension, it is often the case that one or more particle types will have one surface charge while the others will be oppositely charged.  In such cases, oppositely charged particles will attract.  To optimize homogeneity in such cases, high intensity dispersion (HID) should be employed to uniformly distribute all particles.  As soon as the shear is removed, particles will be attracted to those of opposite charge.  If the system is relatively homogeneous during HID, suspension arrangements should be expected to remain homogeneous when the HID is discontinued.

Tests have shown that relatively small additions (hundredths of a percent) of anionic (negatively charged) organic additives can convert all surfaces to negative charges.  Anionic polyelectrolytes which are negatively charged will not only coat positively charged alumina particles, but they will also coat negatively charged silica particles.  So when silica and alumina particles are in the same suspension, small additions of anionic polyelectrolytes render all particle surfaces negative.  At that point, all particles will be similarly charged (negative) so they can be homogeneously mixed and dispersed without concern that they will attach to oppositely charged particles.  Once proper mixing and homogeneity is achieved, other additives can be utilized to further tune the suspensions to achieve desired properties.

Conclusion

In most cases, the real workhorse for tuning suspensions is the control of electrostatic surface charges.  Suspension pH should be tuned first to adjust suspensions into the optimum pH range for the specific types of particles that comprise the suspension.  Then, other additives can be used to adjust suspension properties.

Like-charged particles will repel.  Oppositely charged particles will attract.  Oppositely charged particles (actually, portions of particles) only occur in suspensions of certain single component systems like clays and kaolins.  Oppositely charged particles occur frequently in mixtures of several different types of particles.

Electrostatic surface charges will easily overpower and mask van der Waals attractive forces.  But the hydrophobic effect and organic additives will usually overpower electrostatic forces.  When multiple additives are used in a single suspension, the order of additions and the ultimately desired properties of the suspension must be determined by trial and error. 

High intensity dispersion (HID) can disrupt the fragile equilibria created as additives are used to tune suspensions.  Recognize that HID when used properly, can help to create homogeneous suspensions.  HID, however, can disrupt and change the equilibrium when different types of additives are utilized in a single suspension.  This is not a warning to never combine fundamentally different types of additives.  It is simply a statement of fact -- different types of additives function differently.  When are all present in a single suspension, order of additions, and use or non-use of HID must all be considered.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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